High-frequency surgical devices are increasingly being used in surgery. The generators employed for this purpose supply a fundamental frequency, which typically lies in the range of 300 kHz to 4 MHz. In the generators, power oscillators are provided which rectify and convert electrical energy provided from the main supply network into a potential-free, DC voltage-free output voltage having the aforementioned fundamental frequency. In very many applications, such as high voltage coagulation or coagulating cutting, this fundamental frequency is overlaid with a ‘modulation frequency’, which is typically in the order of 50 kHz. A limited number of sinusoidal oscillations (in an extreme case, a single sinusoidal oscillation) is to be generated, followed by a pulse pause without any energy transfer. Following expiration of a modulation period, the provision of a pulse packet begins again.
It is evident from this that spontaneous starting of the output voltage is required. This means that after 1 or, at most 2, half-periods, the output voltage must have reached its end value, since this influences the effect being striven for on the tissue being treated.
Usually, the energy conversion described is brought about in two successive steps, as illustrated by FIG. 7. First, the electrical energy supplied from the main 1 is rectified by a rectifier 2, so that a constant DC voltage is made available. This constant DC voltage is converted by a power supply 3 (a DC/DC converter) into an intermediate circuit voltage UZ. This intermediate circuit voltage UZ is adjustable. This first energy conversion unit will be referred to in the following as the power supply.
Connected thereto is a second energy conversion unit, which will be referred to in the following as the power oscillator 10. The power oscillator 10 is an inverter, which also includes a potential separation from the patient circuit. The output terminals of the power oscillator 10 are connected, on one side, to an electrosurgical instrument 4 and, on the other side, to a neutral electrode 5. Also connected to the output terminals of the power oscillator 10 is an actual-value sensor 22, which detects the voltage at the output of the power oscillator 10 and compares said voltage with a target voltage from a setpoint generator 21, whereupon the system deviation is fed back to the power supply 3 via a controller 20, so that the intermediate circuit voltage UZ is adjusted such that the output voltage amplitude of the power oscillator 10 is controlled according to the setting of the setpoint generator 21.
As shown in FIGS. 8 and 9, the power oscillator 10 usually includes a driving circuit 11 with power semiconductor devices, a transformer 12, which is enhanced with a parallel connected capacitance CP into a parallel resonant circuit and, at the output of the transformer 12, a series circuit including an inductance LSA and a capacitance CSA, that is, a series resonant circuit toward the patient circuit. In the conventional embodiment shown in FIG. 9, a further series resonant circuit including an inductor LSE and a capacitor CSE is also provided between the output terminals of the driving circuit 11 and the parallel resonant circuit comprising the transformer 12 and the parallel capacitance CP.
In order to ensure the best possible modulation capability and therefore the spontaneous starting of the oscillation of the output voltage, the input series resonant circuit (as per FIG. 9) is often dispensed with (as shown in FIG. 8). The input of the overall resonant circuit is therefore a parallel resonant circuit. Since the driving circuit is also fed from a voltage source, specifically an output capacitance CA of the power supply 3, the driving power semiconductor device 11 represents a short-circuit between a charged capacitor (CA) and an uncharged capacitor (CP). The consequence thereof is that the size of the current flowing is determined only by parasitic manifestations such as lead inductances and path resistances of the semiconductor devices. Since these parasitic values are usually small compared with the actual component values, very high undefined electrical current values arise. These, often very high, pulsed currents can emit undesirable electromagnetic interference (which is not permissible in clinical environments). In addition, the dimensioning of the power semiconductor devices of the driving circuit 11 is often uneconomic.
In order to solve this problem, the circuit shown in FIG. 9 is used, wherein the driving currents are determined by an input inductance LSE. However, this brings with it the disadvantage that the filter does not start to oscillate spontaneously and is therefore not suitable for the aforementioned modulation.